Graded-semiconductor image sensor

ABSTRACT

An image sensor includes a semiconductor material having an illuminated surface and a non-illuminated surface. A plurality of photodiodes is disposed in the semiconductor material to receive image light through the illuminated surface. The semiconductor material includes silicon and germanium, and the germanium concentration increases in a direction of the non-illuminated surface. A plurality of isolation regions is disposed between individual photodiodes in the plurality of photodiodes. The plurality of isolation regions surround, at least in part, the individual photodiodes and electrically isolate the individual photodiodes.

TECHNICAL FIELD

This disclosure relates generally to semiconductor fabrication, and inparticular but not exclusively, relates to composition of semiconductorimage sensors.

BACKGROUND INFORMATION

Image sensors have become ubiquitous. They are widely used in digitalstill cameras, cellular phones, security cameras, as well as, medical,automobile, and other applications. The technology used to manufactureimage sensors has continued to advance at a great pace. For example, thedemands of higher resolution and lower power consumption have encouragedthe further miniaturization and integration of these devices.

Detection of infrared (IR) light is useful in automotive and nightvision applications. However, conventional image sensor devices maypoorly absorb infrared light due to the band structure of semiconductormaterials used in modern microelectronic devices. Even if conventionalimage sensors can absorb IR light, the semiconductor may need to besufficiently thick. Additional semiconductor thickness may complicateother fabrication steps and/or reduce performance.

Furthermore, many materials conducive to absorbing IR light are veryexpensive (either inherently or by virtue of fabrication techniquesneeded to process the materials), toxic, and/or have lower sensitivityto the visible spectrum. Accordingly, many elements/compounds capable ofdetecting IR light may not be ideal choices for integration into modernelectronic devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive examples of the invention are describedwith reference to the following figures, wherein like reference numeralsrefer to like parts throughout the various views unless otherwisespecified.

FIG. 1A is a cross sectional illustration of an example image sensor, inaccordance with the teachings of the present invention.

FIG. 1B is a circuit diagram of the image sensor in FIG. 1A, inaccordance with the teachings of the present invention.

FIG. 2 is a block diagram illustrating one example of an imaging systemincluding the image sensor of FIG. 1A, in accordance with the teachingsof the present invention.

FIG. 3 illustrates an example method for forming the image sensor ofFIG. 1A, in accordance with the teachings of the present invention.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

Examples of an apparatus and method for a graded-semiconductor imagesensor are described herein. In the following description, numerousspecific details are set forth to provide a thorough understanding ofthe examples. One skilled in the relevant art will recognize; however,that the techniques described herein can be practiced without one ormore of the specific details, or with other methods, components,materials, etc. In other instances, well-known structures, materials, oroperations are not shown or described in detail to avoid obscuringcertain aspects.

Reference throughout this specification to “one example” or “oneembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present invention. Thus, the appearances ofthe phrases “in one example” or “in one embodiment” in various placesthroughout this specification are not necessarily all referring to thesame example. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreexamples.

Throughout this specification, several terms of art are used. Theseterms are to take on their ordinary meaning in the art from which theycome, unless specifically defined herein or the context of their usewould clearly suggest otherwise. It should be noted that element namesand symbols may be used interchangeably through this document (e.g., Sivs. silicon); however, both have identical meaning.

FIG. 1A is a cross sectional illustration of an example image sensor100. Image sensor 100 includes: semiconductor material 101, plurality ofphotodiodes 107, plurality of isolation regions 111, and peripheralcircuitry 161. Individual photodiodes 107 in plurality of photodiodes107 include doped region 103 and heavily doped region 105.

Semiconductor material 101 has an illuminated surface 151 and anon-illuminated surface 153. In one example, illuminated surface 151 isa backside of semiconductor material 101, and non-illuminated surface153 is a frontside of semiconductor material 101. Plurality ofphotodiodes 107 is disposed in semiconductor material 101 to receiveimage light through illuminated surface 151, and plurality ofphotodiodes 107 (along with plurality of isolation regions 111) extendthrough a majority of the semiconductor material 101. Semiconductormaterial 101 includes an epitaxially grown silicon and germaniumcompound, and the germanium concentration increases in a direction ofthe non-illuminated surface 153. The increasing germanium concentrationin the direction of the non-illuminated surface 153 results in apotential energy gradient from the illuminated surface 151 to thenon-illuminated surface 153. Potential energy is lower in the directionof the non-illuminated surface 153. An example potential energy gradientis illustrated by the band diagram on the right-hand side of FIG. 1A.Because SiGe has a narrower bandgap than Si, the potential energy ofimage sensor 100 decreases in the direction of non-illuminated surface153 (i.e., the distance between valence band (Ev) and conduction band(Ec) gets smaller approaching the non-illuminated surface 153. In oneexample, the germanium concentration at the non-illuminated surface 153of semiconductor material 101 is greater than 35%. In other examples,the germanium concentration may reach 50% or more. This promotes theflow of free electrons (generated near the illuminated surface 151) tothe non-illuminated surface 153, and helps reduce electrical crosstalkbetween individual photodiodes 107.

To further improve charge transfer from the illuminated surface 151 tothe non-illuminated surface 153, plurality of photodiodes 107 includesdoped region 103 and heavily doped region 105. As depicted, doped region103 is disposed in semiconductor material 101 between illuminatedsurface 151 and heavily doped region 105. Heavily doped region 105 is ofan opposite majority charge carrier type (opposite doping profile) asdoped region 103. In one example, doped region 103 is n-type and heavilydoped region 105 is p-type, while in a different example, doped region103 is p-type and heavily doped region 105 is n-type.

Furthermore, increased Ge content improves infrared (IR) lightsensitivity. While silicon is capable of absorbing IR light, theextinction coefficient of IR light in silicon is lower than theextinction coefficient of visible wavelengths of light. Since SiGe has anarrower bandgap than native silicon, IR light is readily absorbed inrelatively thin SiGe layers (e.g., less than 2.5 μm thick). In thedepicted example, blue light is absorbed at a first depth insemiconductor material 101, green light is absorbed at a second depth insemiconductor material 101, red light is absorbed at a third depth insemiconductor material 101, and infrared light is absorbed at a fourthdepth (where the Ge concentration is high) in semiconductor material101. The fourth depth is greater than the third depth, the third depthis greater than the second depth, and the second depth is greater thanthe first depth. Furthermore, depth is measured from illuminated surface151.

As shown, plurality of isolation regions 111 is disposed betweenindividual photodiodes 107 in the plurality of photodiodes 107.Plurality of isolation regions 111 surround, at least in part,individual photodiodes 107 and electrically isolate individualphotodiodes 107. Although FIG. 1A depicts a partial cross sectional viewof image sensor 100, isolation regions 111 encircle individualphotodiodes 107 and include doped portions of semiconductor material101. In other examples, isolation regions 111 may be etched intosemiconductor material 101 and include metals, and/or oxides.

In the depicted example, peripheral circuitry 161, including controlcircuitry and readout circuitry, is laterally proximate to plurality ofphotodiodes 107 and is electrically coupled to plurality of photodiodes107. Although in the depicted example, peripheral circuitry 161 isdisposed on the left-hand side of the plurality of photodiodes 107, inother examples, peripheral circuitry 161 may be disposed on any side(s)of plurality of photodiodes 107, or may entirely surround plurality ofphotodiodes 107. Additionally, as shown peripheral circuitry 161 may beseparated from plurality of photodiodes by deep trench isolation 135 toprevent electrical crosstalk. Deep trench isolation 135 may be filledwith semiconductor, oxide, or metal materials.

FIG. 1B is a circuit diagram of image sensor 100 in FIG. 1A. In thedepicted example, image sensor 100 includes: semiconductor material 101,plurality of photodiodes 107, plurality of transfer gates 113, floatingdiffusion 121, reset transistor 123, amplifier transistor 131, and rowselect transistor 133. Plurality of photodiodes 107 is disposed insemiconductor material 101 to accumulate image charge in response toincident light directed into plurality of photodiodes 107. In oneexample, semiconductor material 101 may include silicon and germanium,but may include other suitable semiconductor materials and dopant atoms.Plurality of transfer gates 113 is also disposed in semiconductormaterial 101 and individual transfer gates 113 in plurality of transfergates 113 are coupled to individual photodiodes 107 in plurality ofphotodiodes 107. Floating diffusion 121 is disposed in semiconductormaterial 101, and floating diffusion 121 is coupled to the plurality oftransfer gates 113 to receive image charge from plurality of photodiodes107 in response to a transfer signal sequentially applied to a controlterminal of each individual transfer gate 113. In other words, in thedepicted example, a transfer signal is applied to the control terminalof the top transfer gate 113, then a transfer signal is applied to thecontrol terminal of the second-from-the-top transfer gate 113, etc.Reset transistor 123 is coupled to floating diffusion 121 to extract theimage charge from floating diffusion 121. Further, amplifier transistor131 is coupled to floating diffusion 121, and row select transistor 133is coupled between an output of amplifier transistor 131 and a bit lineoutput. In one example, amplifier transistor 131 includes a sourcefollower coupled transistor.

Although not depicted in FIG. 1A, elements of readout circuitry may bedisposed proximate to the non-illuminated surface 153 of semiconductormaterial 101. For example, transfer gates 113 and floating diffusion 121may be disposed near the non-illuminated surface 153 to readout imagecharge from the plurality of photodiodes 107. Accordingly, theincreasing germanium concentration in the direction of thenon-illuminated surface 153 is useful to consolidate charge near theactive circuit elements. Charge consolidation may make electron/holeextraction from plurality of photodiodes 107 more efficient.

In the depicted example, plurality of photodiodes 107 includes fourphotodiodes 107 coupled to floating diffusion 121 through transfer gates113. However, in a different example, any number of photodiodes 107 maybe coupled to floating diffusion 121 including two, six, and eightphotodiodes 107. In the depicted example, the four photodiodes 107include one photodiode 107 disposed to absorb green light, onephotodiode 107 disposed to absorb blue light, one photodiode 107disposed to absorb red light, and one photodiode 107 disposed to absorbinfrared light. Although not depicted in FIG. 1B, color selection may beaccomplished by placing a color filter layer proximate to semiconductormaterial 101. In one example, the color filter layer includes red,infrared, green, and blue color filters which may be arranged into aBayer pattern, EXR pattern, X-trans pattern, or the like. However, in adifferent or the same example, the color filter layer may includeinfrared filters, ultraviolet filters, or other light filters thatisolate invisible portions of the EM spectrum. In the same or adifferent example, a microlens layer is formed on the color filterlayer. The microlens layer may be fabricated from a photo-active polymerthat is patterned on the surface of the color filter layer. Oncerectangular blocks of polymer are patterned on the surface of the colorfilter layer, the blocks may be melted (or reflowed) to form thedome-like structure characteristic of microlenses.

FIG. 2 is a block diagram illustrating one example of imaging system 200including the image sensor of FIG. 1A. Imaging system 200 includes pixelarray 205, control circuitry 221, readout circuitry 211, and functionlogic 215. In one example, pixel array 205 is a two-dimensional (2D)array of photodiodes, or image sensor pixels (e.g., pixels P1, P2 . . ., Pn). As illustrated, photodiodes are arranged into rows (e.g., rows R1to Ry) and columns (e.g., column C1 to Cx) to acquire image data of aperson, place, object, etc., which can then be used to render a 2D imageof the person, place, object, etc. However, photodiodes do not have tobe arranged into rows and columns and may take other configurations.

In one example, after each image sensor photodiode/pixel in pixel array205 has acquired its image data or image charge, the image data isreadout by readout circuitry 211 and then transferred to function logic215. In various examples, readout circuitry 211 may includeamplification circuitry, analog-to-digital (ADC) conversion circuitry,or otherwise. Function logic 215 may simply store the image data or evenmanipulate the image data by applying post image effects (e.g., crop,rotate, remove red eye, adjust brightness, adjust contrast, orotherwise). In one example, readout circuitry 211 may readout a row ofimage data at a time along readout column lines (illustrated) or mayreadout the image data using a variety of other techniques (notillustrated), such as a serial readout or a full parallel readout of allpixels simultaneously.

In one example, control circuitry 221 is coupled to pixel array 205 tocontrol operation of the plurality of photodiodes in pixel array 205.For example, control circuitry 221 may generate a shutter signal forcontrolling image acquisition. In one example, the shutter signal is aglobal shutter signal for simultaneously enabling all pixels withinpixel array 205 to simultaneously capture their respective image dataduring a single acquisition window. In another example, the shuttersignal is a rolling shutter signal such that each row, column, or groupof pixels is sequentially enabled during consecutive acquisitionwindows. In another example, image acquisition is synchronized withlighting effects such as a flash.

In one example, imaging system 200 may be included in a digital camera,cell phone, laptop computer, automobile or the like. Additionally,imaging system 200 may be coupled to other pieces of hardware such as aprocessor (general purpose or otherwise), memory elements, output (USBport, wireless transmitter, HDMI port, etc.), lighting/flash, electricalinput (keyboard, touch display, track pad, mouse, microphone, etc.),and/or display. Other pieces of hardware may deliver instructions toimaging system 200, extract image data from imaging system 200, ormanipulate image data supplied by imaging system 200.

FIG. 3 illustrates an example method 300 for forming the image sensor ofFIG. 1A. The order in which some or all process blocks appear in method300 should not be deemed limiting. Rather, one of ordinary skill in theart having the benefit of the present disclosure will understand thatsome of method 300 may be executed in a variety of orders notillustrated, or even in parallel. Furthermore, method 300 may omitcertain process blocks in order to avoid obscuring certain aspects.Alternatively, method 300 may include additional process blocks that maynot be necessary in some embodiments/examples of the disclosure.

Process block 301 illustrates providing a silicon layer. In one example,the silicon layer may be a silicon wafer for use as a substrate toepitaxially grow the semiconductor material via chemical vapordeposition, atomic layer deposition, molecular beam epitaxy or the like.In one example, the silicon layer may be a silicon buffer layer.

Process block 303 shows epitaxially growing the SiGe semiconductormaterial on the silicon layer. In one example, this involves depositinga layer of antimony on the silicon buffer layer and growing thesemiconductor material (SiGe) such that the layer of antimony isdisposed between the silicon buffer layer and the semiconductormaterial. The germanium concentration increases in the direction of thenon-illuminated surface of the semiconductor material. In one example,the germanium concentration at the non-illuminated surface of thesemiconductor material is greater than 35%, and the semiconductormaterial is less than 2.5 μm thick.

Although using SiGe as the semiconductor material is discussed in depththroughout this application, other narrow bandgap semiconductormaterials may be employed including: GaAs, Pbs, PbSe, PbTe, GaSb, InN,etc. Furthermore, any group 3 elements (B, Al, Ga, In, Tl), group 4elements (C, Si, Ge, Sn, Pb), group 5 elements (N, P, As, Sb, Bi), orthe like, may be used to form a suitable IR-absorbing semiconductormaterial, in accordance with the teachings of the present invention.

Process block 305 depicts forming a plurality of photodiodes (includinga doped region and a heavily doped region) in the semiconductor materialvia ion implantation. However in other examples, the plurality ofphotodiodes is formed by dopant inclusion during semiconductor materialgrowth (e.g., incorporating arsenic-based gasses in a chemical vapordeposition semiconductor growth process). In the depicted example, thedoped region is of an opposite majority charge carrier type as theheavily doped region, and the doped region is disposed between theilluminated surface and the heavily doped region.

Process block 307 shows forming (via ion implantation) a plurality ofisolation regions disposed between individual photodiodes in theplurality of photodiodes. The plurality of isolation regions surround,at least in part, the individual photodiodes and electrically isolatethe individual photodiodes. Isolation regions may include a number ofsemiconductor elements and dopants. In some examples, isolation regionsmay also include oxides/nitrides such as silicon oxide (SiO₂), hafniumoxide (HfO₂), silicon nitride (Si₃N₄), silicon oxynitride(SiO_(x)N_(y)), tantalum oxide (Ta₂O₅), titanium oxide (TiO₂), zirconiumoxide (ZrO₂), aluminum oxide (Al₂O₃), lanthanum oxide (La₂O₃),praseodymium oxide (Pr₂O₃), cerium oxide (CeO₂), neodymium oxide(Nd₂O₃), promethium oxide (Pm₂O₃), samarium oxide (Sm₂O₃), europiumoxide (Eu₂O₃), gadolinium oxide (Gd₂O₃), terbium oxide (Tb₂O₃),dysprosium oxide (Dy₂O₃), holmium oxide (Ho₂O₃), erbium oxide (Er₂O₃),thulium oxide (Tm₂O₃), ytterbium oxide (Yb₂O₃), lutetium oxide (Lu₂O₃),yttrium oxide (Y₂O₃), or the like. Additionally, one skilled in therelevant art, will recognize that any stoichiometric combination of theabove metals/semiconductors and their oxides/nitrides/oxynitrides may beused, in accordance with the teachings of the present invention.

Block 309 depicts forming peripheral circuitry, including controlcircuitry and readout circuitry, laterally proximate to the plurality ofphotodiodes and electrically coupled to the plurality of photodiodes (asillustrated in FIG. 1A). Furthermore, process block 309 illustratesforming an optical stack. The optical stack may include color filters,microlenses and other secondary optical structures to optimize imageacquisition by the image sensor.

The above description of illustrated examples of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific examples of the invention are described herein forillustrative purposes, various modifications are possible within thescope of the invention, as those skilled in the relevant art willrecognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific examples disclosedin the specification. Rather, the scope of the invention is to bedetermined entirely by the following claims, which are to be construedin accordance with established doctrines of claim interpretation.

1. An image sensor, comprising: a semiconductor material having anilluminated surface and a non-illuminated surface, and a plurality ofphotodiodes arranged into an array and disposed to receive image lightthrough the illuminated surface, wherein a portion of the semiconductormaterial, including the plurality of photodiodes arranged into thearray, includes silicon and germanium, and a germanium concentrationincreases in a direction of the non-illuminated surface to increase redand infrared sensitivity of the plurality of photodiodes, whereinincreasing the germanium concentration in the direction of thenon-illuminated surface results a potential energy gradient from theilluminated surface to the non-illuminated surface, and whereinpotential energy is lower in the direction of the non-illuminatedsurface; a plurality of isolation regions disposed between individualphotodiodes in the plurality of photodiodes, wherein the plurality ofisolation regions surround, at least in part, the individual photodiodesand electrically isolate the individual photodiodes; and peripheralcircuitry disposed in a peripheral region laterally proximate to theplurality of photodiodes and separated from the array by a deep trenchisolation structure, wherein the peripheral circuitry includes, at leastin part, control circuitry to control operation of the plurality ofphotodiodes and readout circuitry to readout image charge from theplurality of photodiodes, wherein the peripheral region is substantiallysilicon.
 2. (canceled)
 3. The image sensor of claim 1, wherein bluelight is absorbed at a first depth in the semiconductor material, greenlight is absorbed at a second depth in the semiconductor material, redlight is absorbed at a third depth in the semiconductor material, andinfrared light is absorbed at a fourth depth in the semiconductormaterial, wherein the fourth depth is greater than the third depth, thethird depth is greater than the second depth, and the second depth isgreater than the first depth, and wherein depth is measured from theilluminated surface.
 4. The image sensor of claim 1, wherein thegermanium concentration at the non-illuminated surface of thesemiconductor material is greater than 35%.
 5. The image sensor of claim1, wherein the plurality of isolation regions encircle the individualphotodiodes and include doped portions of the semiconductor material. 6.The image sensor of claim 1, wherein each one of the plurality ofphotodiodes includes a doped region and a heavily doped region, whereinthe heavily doped region is of an opposite majority charge carrier typeas the doped region, and wherein the doped region is disposed in thesemiconductor material between the illuminated surface and the heavilydoped region.
 7. (canceled)
 8. The image sensor of claim 1, wherein theplurality of photodiodes and the plurality of isolation regions extendthrough a majority of the semiconductor material.
 9. The image sensor ofclaim 1, wherein the semiconductor material is less than 2.5 μm thick.10. (canceled)
 11. The image sensor of claim 1, wherein at least aportion of the semiconductor material containing silicon and germaniumis epitaxially grown. 12-20. (canceled)
 21. The image sensor of claim 1,further comprising: a plurality of transfer gates disposed proximate tothe non-illuminated surface of the semiconductor material and coupled tothe plurality of photodiodes, wherein individual transfer gates in theplurality of transfer gates are coupled to individual photodiodes in theplurality of photodiodes; a floating diffusion coupled to receive theimage charge from the plurality of photodiodes in response to a transfersignal sequentially applied to a control terminal of the individualtransfer gates; a reset transistor coupled to reset the image charge onthe floating diffusion; a source follower transistor coupled to thefloating diffusion to amplify the image charge on the floatingdiffusion; and a row select transistor coupled to an output terminal ofthe source follower transistor and a bit line output.
 22. The imagesensor of claim 21, wherein four photodiodes in the plurality ofphotodiodes are coupled to the floating diffusion.
 23. The image sensorof claim 21, wherein the plurality of transfer gates are positioned toextract the image charge from a location of consolidated charge in theplurality of photodiodes proximate to the non-illuminated surface,resulting from the germanium concentration increasing in a direction ofthe non-illuminated surface.
 24. The image sensor of claim 1, whereinthe deep trench isolation structure separating the plurality ofphotodiodes arranged into an array from the peripheral circuit regionincludes at least one of an oxide or a metal.
 25. The image sensor ofclaim 24, wherein the deep trench isolation structure entirely surroundsthe plurality of photodiodes arranged into an array to prevent opticalcrosstalk between the substantially silicon based peripheral circuitryand the plurality of photodiodes.
 26. The image sensor of claim 1,further comprising a color filter layer disposed proximate to thesemiconductor material and positioned so that a first photodiode in theplurality of photodiodes absorbs blue light from a blue color filter, asecond photodiode in the plurality of photodiodes absorbs green lightfrom a green color filter, a third photodiode in the plurality ofphotodiodes absorbs red light from a red color filter, and a fourthphotodiode in the plurality of photodiodes absorbs infrared light froman infrared color filter.
 27. The image sensor of claim 1, furthercomprising a layer of antimony disposed in the semiconductor material.28. The image sensor of claim 1, wherein the plurality of isolationregions include at least one of hafnium oxide, tantalum oxide, zirconiumoxide, titanium oxide, or lanthanum oxide.